1. Field of Invention
The present invention relates to the automated unitization of multiple products into single loads that will maintain their unitary integrity throughout shipment. More specifically, this invention relates to a method and apparatus for dispensing and wrapping a stretchable film around a plurality of stacked units located on a rotatable turntable.
2. Description of the Prior Art
Automatic unitization of loads is currently being accomplished by various types of machines. Specific examples of such machines include strapping and string-tying equipment; automatic taping machines; palletizers that glue the products together by automatically dispensing hot melt between the units as they are being palletized; automatic shrink film placing units that work in conjunction with shrink tunnels; and automatic dual roll push through types stretch film wrappers.
The currently used automatic stretch film placing units stretch the entire web of film at the same time to force it around the load and heat seal the ends of the film together. This apparatus has the disadvantage of requiring enormous forces to stretch the film. In addition, the apparatus pulls the film across the corners of the load which may cause rupturing of the film or deformation of the load. Most importantly, the process requires a critically important seal to reliably secure the load during shipment. If the heat sealing jaws of the apparatus do not provide a perfectly uniform pressure and temperature along their entire length then the film seal will not be particularly strong. Thus the seal will be overheated in some areas which will melt the film thin, or crystalize it to a state of brittleness. Alternatively if the seal is underheated in some areas deterioration or unwrapping may occur under the tension of shipment.
In the present invention, the placing of a thin gauge film around the load in multiple layers requires considerably smaller forces to be used in order to obtain the film elongation as is required. As the load is rotated on a turntable, the application of the film to the load eliminates any sliding or abrading of the film as conventionally occurs. In the invention no seals are required when "tacky" film is utilized, as this type of film normally will have enough cling to bond to itself. In those instances where greater fastening is required, a simple tack seal bonds the trailing edge of the film to the layers already disposed about the load. As with the other types of machines the tack seal is completely automatic, which is a necessity in modern plants.
The main advantage of the present invention over the automatic strap, string and taping machines is that this invention spreads the unitizing forces over the product so that the forces are not concentrated at specific points which often tends to deform the product. The conventional solution of this problem is for padding to be placed beneath the straps. This placement is invariably performed by hand and thus defeats the automatic feature of the operation.
The prior art also discloses the glueing of bags or boxes in order to unitize multiple products into a single load. The glueing is frequently objectionable to the customers who must pull it apart and does not work particularly well when the product is column stacked.
The automatic improvement of the invention centers around the clamping and cutting technique. As the film-holding clamps are wrapped and removed prior to final revolution, the clamps can be positioned off the side of the conveyor thereby eliminating the possibility of the pallet load hitting the clamps. Additionally, the clamping and cutting process of the invention eliminates reliance on the pallet configuration or location to insure proper cutting action and clamping.
This invention is an improvement over the U.S. Pat. No. 3,867,806 in which a process for making a unitary package is disclosed. In the patent a load comprising a plurality of units is formed by wrapping a band formed of plural layers of a stretched material around the units. This patent requires the holding of the film's leading edge against the vertical surface of the package load by either manual or other non-disclosed fastener means. The present invention specifically discloses clamps located both on the rotatable conveyor and the top platen which serve to hold the leading edge of the film against the vertical side of the load to be wrapped. After the first layer of film is wrapped around the load and clamps, the clamps located on the platen and conveyor are placed in their retracted positions so that the load can be conveyed without interference from the clamps.
Upon completion of the fastening steps disclosed in the U.S. Pat. No. 3,867,806, the operator must sever the overwrapped package from the film dispenser by employing a drop-knife cutter means which is mounted adjacent the pallet support means. In the present invention, film clamp/cutter jaws are used which serve to hold the film in a vertical position while a toothed film cutting bar pierces the entire vertical length of the film at the same instant. In both inventions, the elongation of the film is due to the tension imparted thereon by a tension brake acting on the dispensing roll. Prior to separation the amount of elongation is uniform along the vertical length of the package. In the prior art mode of cutting the film by a sliding cutting knife, the tension is released and consequently the elongation decreased as the cutting blade passes through the film. Thus the amount of elongation at the top of the film through which the blade has passed will be less than the elongation present at the bottom of the film, where the blade has yet to pass. This lack of uniformity can lead to the load having a non-uniform stretched film. Consequently a less functional wrap is produced which in addition is less aesthetically appealing. Such appearance is critical in clear plastic wrapped packages. This is so in spite of the fact that the cutting operation is performed subsequent to the sealing operation. Conversely, the present invention cuts the entire length of the film at one time, thereby keeping the amount of tension and percent of elongation constant and uniform alongthe length of the film. The uniformity of tension provides a more functional and aesthetically pleasing package. The effect of a onetime total length cut is further enhanced by the fact that the present invention utilizes a tacky film. This particular type of film does not require any additional sealing means other than the wipe operation of a brush which secures the trailing edge to the vertical side of the package.
U.S. Pat. No. 3,514,920 discloses an apparatus for overwrapping a package by means of a heat shrink film shrunk around a load to provide a unitary package. In that patent the heat-shrinkable plastic film used as an overwrap for the package has a substantially rectangular configuration and has a leading edge portion fastened against the cartons. It will be noted that this apparatus specifically requires the operator to attach the leading edge of the plastic film to the cartons. The present invention, as previously mentioned eliminates this manual operation. In addition to holding the leading edge of the film against the load, the present invention comtemplates the performance of a cutting operation. After the next load is in position, the wrapping clamps grab and hold the new leading edge of the film in order to facilitate the packaging of the pallet and associated load.
The present invention eliminates the use and excessive cost of heat tunnels since the film used is stretchable and consequently no heat is required to secure the package.
U.S. Pat. No. 3,793,798 discloses a shrink palletized process and apparatus. That particular package and process also specifically contemplates the manual withdrawing of the leading edge of the film and holding that leading edge against the package. The present invention, however, performs this task automatically.
An additional advantage of the present invention is the use of the upper platen clamp in conjunction with lower conveyor located clamp. The utilization of the two clamps allows the film to be held firmly against the load at two points. This allows the package to be wrapped uniformly with equal film tension and elongation along the entire length of the vertical sides of the load.
In previously developed stretch-film wrapping machines, an edge of the film is either tacked or sealed to the previously wrapped film layers prior to the time of film cutting or separation. This is required because it is necessary to maintain the backward tension and consequently the elongation of the film prior to separation in order for a tight seal to be produced. After the seal is formed, the film is cut. This leaves a portion of the film i.e. between the seal and the trailing edge as wasted film. The present invention avoids this waste of film by cutting the film and subsequently wiping the film against the vertical side of the load thereby utilizing the entire film length in a functional capacity.
Other wrapping patents of general interest are U.S. Pat. Nos. 3,795,086 and 3,589,102. Patents which disclose the spiral wrapping of articles and are of interest are U.S. Pat. Nos. 2,575,467, 3,003,297, 3,788,199 and 3,863,425.